One of the fundamental concepts consists in forming, in silicon, optical waveguides (lightguides) having very small dimensions (a few hundred nanometres in cross section) subjected to the influence of variable concentrations of carriers (electrons or holes) in the silicon. By electrically modulating the carrier concentration, it is possible to vary the refractive index of the optical medium of the waveguide and therefore the speed of propagation of the light in the waveguide is varied.
For example, by modulating the propagation time of the light in two different optical branches, which are separately controlled, a phase shift is produced between the optical waves leaving the two branches. By making the waves interfere with each other, this phase shift is converted into an amplitude modulation: this is the principle of Mach Zehnder interferometers.
Notably, silicon waveguides called slot waveguides have been devised which comprise two doped silicon walls (of high refractive index), separated by a slot filled with an insulating, or sometimes semi-insulating, medium of low refractive index. The light is confined in the waveguide because of the index discontinuities between the silicon walls and the material of the slot. Confinement is all the better the narrower the slot and the higher the index difference. The two walls may be doped with impurities of the same type and the waveguide behaves electrically as a capacitor. The voltage across the terminals of the capacitor creates electrical charge concentrations in the silicon walls and the concentration variations (whether these be passive or depend upon an electrical control) generate refractive index variations. However, the silicon walls may also be doped with impurities of opposite type, the waveguide then behaving as a pn junction and the voltage variations across the terminals of the junction also create accumulations or depletions of electrical charges acting on the refractive index, and therefore on the light propagation. The insulator (or semi-insulator) placed between the two silicon walls constitute an electrical barrier preventing the transport of charge carriers between the walls. The article “Silicon optical modulators” by G. T. Reed, G. Mashanovich, F. Y. Gardes and D. J. Thomson in Nature Photonics, Vol. 4, August 2010, pages 518-526, describes optical modulators on silicon using slot waveguide structures.
With such slot waveguides, it is possible to produce optical switches, light sources and optical sensors.
It has already been proposed to produce such waveguides by excavating a slot in a silicon layer and filling this slot with insulating or semi-insulating material. However, this process cannot be used to make very narrow slots of sufficient height, since the filling operation cannot take place properly.
It has also been proposed, in patent application of FR 2 907 916, to produce slot waveguides on an SOI (Silicon On Insulator) substrate. The SOI substrate is a silicon substrate covered with an insulating layer (of silicon oxide) which is itself covered with an active silicon layer. In the above publication, a layer of non-stoichiometric silicon oxide SiOx is firstly deposited, this layer is etched to form two parallel trenches close to each other separated by an insulating or semi-insulating wall, and the silicon trenches are then filled. The silicon may be amorphous or polycrystalline silicon. However, amorphous or polycrystalline silicon is much inferior to single-crystal silicon from the standpoint of the expected effect, which is to obtain rapid and well-defined variations of the free carriers in the silicon. To fill the single-crystal silicon trenches, the SOI substrate must be covered beforehand with a layer of single-crystal silicon before deposition of the SiOx layer. This single-crystal layer may then serve as a nucleus to grow single-crystal silicon in the open trenches in the SiOx layer.
However, the process described in that publication has the major drawback of not providing sufficient electrical isolation between the two silicon walls, since they are connected at their base because they both rest on the same single-crystal silicon layer. Even if this layer is very slightly doped and very thin, it is not completely insulating and carriers can move therein, even more so when the slot between the two walls is very narrow.